Ultraviolet detector system with means to keep electrodes contamination-free



J1me 1955 J. B. JOHNSON ETAL 3,191,036

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FIG. 2 fi/zo "r0 ALARM v CIRCUIT w INVENTORS JOHN B. JOHNSON ALBERT LEENUnited States Patent 3,191,036 ULTRAVlOLET BETECTGR SYSTEM WITH MEANS TOKEEP EEESTRGDES 0NTAMHATIGN$REE John I .l'ohnson, Miilhurn Township,Essex County, and

Albert Leen, West Caldwell, NJ assignors to McGraw- Edison Company,Elgin, 112., a corporation of Delaware Continuation of application Ser.No. 83,755, Jan. '19, 1961. This application Why 10, 1963:, Ser. No.280,193 6 Claims. (Ci. fill-83.6)

This application is a continuation of our application Serial No. 83,755,filed January 19, 1961, and being abandoned upon the filing of thisapplication.

This invention relates to ultraviolet detector systems utilizingdetector tubes of the'type described in the pending Howling applicationSerial No. 801,625, filed March 24, 1959 (now Patent No. 3,047,761,dated July 31, 1962). More particularly, the invention relates to anovel combination of such detector tube with an electrical operatingcircuit which causes the tube to have a more stable response.

An object of the invention is to provide improvements in ultravioletdetector systems of the character mentioned which are adapted tomaintain the systems stable in their operation especially as to theirspectral response.

A further object is to provide special operating circuits for thedetector tube abovementioned, which are designed to subject the tube tooperating conditions effective to cause the tube to have a uniform andstable response.

A further object is to provide improved means and methods of maintainingthe electrodes of the aforesaid tube clean of low work functioncontaminants such as would alter the spectral response of the tube.

The above tube is operated in either an AC. circuit or in a DO pulsingcircuit so that the tube is quenched by the drop of the voltage at theend of each voltage pulse, thereby enabling the tube to be operated in alow impedance circuit for maximum power efficiency. Preferably, the tubeis provided with symmetrical electrodes and is operated in an A.C.circuit so that it will be triggered and conductcurrent during some orall half cycles so long as the tube is subjected to incident photonssutlicient to trigger a discharge of the tube. The tube electrodes arecharacterized as being free of sharp edges, projections or otherdiscontinuities as well as of any contaminants throughout the workingregion, this being the entire region wherein an emitted electronresponsive to an incident photon is capable of triggering an avalanchedischarge between the electrodes. However, it has been found that duringuse of the tubes contaminants such as alkali metal atoms are releasedfrom the glass envelopes when commercial glass materials are used, andthat these contaminants will lodge on the electrodes within the workingregion and there lower the work function and cause the tubes to respondrandomly to ultraviolet radiation of longer wave length than wasintended. This random response is particularly objectionable as to tubeswhich 'are designed to be solar blind.

It has been found'in accordance with the invention that if each glowdischarge covers the electrodes throughout the entire working region thecontaminants are removed by ion bombardment and the spectral responseremains stable throughout the life of the tubes without the tubesbecoming responsive to radiation in the solar spectrum. However, if theglow discharge covers the entire working region for the duration of eachvoltage pulse of the supply circuit, the electrodes will overheat andemit' electrons thermionically to cause the tubes to lock-on i.e., failto quench at the end of the voltage. pulses after the incident photonradiation has ceased. This difficulty has however been coped withsuccessfully in i lfi h Fate-rated June 22, 15965 ice accordance withthe present invention by using an operating circuit which causes theglow to cover the entire working region for at least an instant of eachdischarge without the R.M.S. current reaching such value that wouldoverheat the electrodes to the point of producing thermionic emission.As will appear, these conditions have been met by suitably operatingcircuits according to the invention to provide a stable operationthroughout the life of the tube.

These and other objects and features of the invention will be apparentfrom the following description and the appended claims.

In the description of the invention, reference is bad to accompanyingdrawings of which:

FIGURE 1 is a schematic circuit diagram of an illustrative operatingcircuit for the above detector tube wherein the tube is operated from anAC. voltage source;

FIGURE 1a is a fractional drawing of the circuit of FIGURE 1 showing amodification thereof; and

FIGURE 2 is a schematic circuit diagram of an illustrative operatingcircuit for the above detector tube wherein the tube is operated by DC.pulses.

The ultraviolet detector tube 10 shown in the figures comprisespreferably two tungsten-wire electrodes 11 having semicircular endportions directed away from each other and welded to supporting pins 12and having rectilinear intermediate portions parallel to each other. Asan example, the supporting pins and tungsten wires may be .050 and .017"respectively in diameter and the spacing between the rectilinearportions of the tungsten wires may be .040". The tungsten wires,supporting pins and weld joints are electropolished and refined asdescribed in the aforesaid Howling patent so that they have ultrasmoothsurfaces free of edges, projections or other discontinuities as well asof any contaminants. It is particularly important that the weld jointsto the supporting pins be smooth, round and free of contaminants inorder to confine the working region to a space encompassing only theparallel portions and portions of the half-circle end formations of thetungsten-wire electrodes.

The tube is provided with a commercial ultraviolet transmitting glasssuch as of the borosilicate family, for example, Corning glass No. 9741.The supporting pins 12 are extended through graded glass seals in thebase of the envelope. The envelope 10 is pumped to a high vacuum whilethe tungsten wire electrodes are heated, the envelope is filled with asubstantially pure hydrogen to a pressure typically of about 10 cm. Hgto obtain a striking voltage of about 700 volts under ultravioletexcitation and, finally, the tungsten-wire electrodes are subjected topositive ion bombardment to remove any trace of impurity.

A result of the refining operations carried out in the construction ofthe above ultraviolet tube is that all elemental surface areas of theelectrodes within the working region are made to have a uniformly highwork function causing the tube to have a spectral response sensitiveonly to radiations of wave lengths below about 2800 to 3000 Angstromunits. Thus, the tube is inherently highly responsive to ultravioletradiation but unresponsive to sunlight in the normal atmosphere. 7

It is found however that the commercial ultraviolet transmitting glassescontain alkali metals as a fundamental ingredient which appear to resideas ions in the interstices of the open lattice strucure of the silica,boron, oxygen and other ingredients of the glasses. These alkali metalions are released into the gas space under influence of the heat of ionbombardment and of the electric fields occurring during the discharge.Some of these alkali metal ions will lodge on the electrode surfaces inthe working region and there. reduce the work function to provideincremental surface areas responsive to ultraviolet radiation of longerwave length within the solar spectrumi.e.,

to wave lengths above 2800v to 3000 Angstrom units. Al though thesecontaminants can be largely avoided by using an alkali-free glass forthe envelope such as a pure fused quartz, such procedure is not aneconomical one and would leave still a source of contaminantsfrom thegraded glass seals where the supporting 'pins'are brought intotheenvelope.

During the course of making numerous tests (in the above ultravioletdetector tube it was observed that under certain operating conditionsthe tubes had'a more stable response with less tendency to be sensitiveto the longer wave length radiation. For example, tubes which-had beenoperated so that the glow of ionized gas covered'the entire workingregion'of the electrodes, 'or which were operated under relativelyintense unidirectional current pulses were found to remain relativelyinsensitive to the longer wave length radiation. However, tubes whichwere operated under such relatively intense ALC. current would lock-on.These observations led to the realization that a glow discharge has theelfect of cleaning the electrode surface by ionic bombardment and thatupon extending the glow discharge throughout the working-region thetubes will have a stable response but that the R.M.S. value of thecurrent flow through the tube must be limited so that the electrodeswill not overheat and emit electrons causing lock-on by thermionicemission. Further tests have shown that high level instantaneous spikesof current sulficient to extend the glow discharge throughout the entireworking region are s'ufiicient to keep the electrodes clean of'low workfunction contaminants collected during the previous pulse and/ or duringthe quenched condition, and that such instantaneous high level spikes ofcurrent will not overheat the electrodes and cause thermionic emissionso long as the R.M.S. value of the current is below the criticalthreshold value.

A preferred operating circuit for maintaining the ultraviolet tube 10during its operation with a stabilized response is the AC. circuit 14-shown in FIGURE 1. This circuit is connected to a potential source 15through a stepup transformer 16 so that the circuit will receive anR.M.S. voltage of the order of 700 volts. Connected serially in theoperating circuit is a condenser 17, resistor 18, the ultravioletdetector tubeli) and a full-wave rectifier 19. The output of therectifier is connected to a relay shunted by a condenser 21. Typicalcomponent values are as follows: condenser 17, .075 mfd.; resistor 18,3600 ohms; and condenser 21, 50 mfd. Preferably the resistor 18 isconnected in the circuit in close physical relationship to theultraviolet tube 10 so as to prevent a flow of stray capacity currentwhich may quench the discharge in the ultraviolet tube and preventoperation of the relay.

Upon the application of a voltage pulse across the circuit 14 from thesource 15 and the simultaneous triggering of the ultraviolet tube byincident photons, an initial current spike will flow due to the suddencollapse of the voltage across the detector tube from 700 or more voltsto 330 volts. This current spike has a value determined by the resistor18 since the condenser 17 does not at the outset support any voltage.The resistor 18 is chosen so that the initial current spike will producea glow discharge in the tube 10 throughout the entire working region,and the RC time constant of the condenser 17 and resistor 18 is chosenso that each current pulse will decay rapidly enough to preventoverheating of the electrodes over a continued period of ultravioletexcitation of the tube. Although the time interval of each half cycle ofapplied voltage is 8.3 milliseconds for a 60 c.p.s. source of voltage, aseries RC time constant of only .27 millisecond is found highlysatisfactory in preserving the stability of the spectral response of thetube while providing still an ample output current of the order of 10milliamps R.M.S.

. at saturation. The bridge rectifier 19 provides full waverectification of the discharge pulses for the D.C. relay i. 20, and thecondenser 21 smooths out the succesive pulses into a steady current.

In a more specific sense it may be noted that as the supply voltagerises in the circuit 14 from zero a nearly equal counter voltage buildsup across the ultraviolet tube 10 because until the tube is fired it isthe highest impedance element in the circuit. When the counter voltageacross the tube 10 reaches the firing voltage of approximately 700volts, the tube discharges if there isincident photon radiation totrigger it. The instant a discharge occurs in the ultraviolet tube 10the voltage across the tube-falls to approximately 330 volts. Theinitial current is determined by the resistor 18 and the net voltage inthe circuit across this resistor. The instant the current begins to flowa charge begins toybuild up on the condenser 17 causing a potential tobuild up across the condenser and causing the current to begin to decay.When the source potential falls below 330 volts the tube is quenched. Atthis instant there is a potential across the condenser 17 in a'directionto aid the discharge in the next half cycle. For the values of thecircuit components given above, the peak value of the current spikes inthe succeeding half cycles reach a value of the order of 1.85 ampere.

Because of the increasing voltage which becomes applied across theultraviolet tube 10 after the first pulse as described above, there is apossibility of multiple countsa condition wherein the probability of apulse on the next half cycle is greater than that on the first halfcycle for the same radiation excitation. A means of reducing thistendency towards multiple counting while still retaining the advantageof the condenser 17 in maintaining a stable spectral response of thetube without incurring overheating of the electrodes, is in connecting aresistor 22 of appropriate value in parallel with the condenser 17 asshown in FIGURE 1a. This resistor bleeds off some of the charge on thecondenser during the period between pulses to mitigate the possibilityof multiple counts. A suitable time constant for the RC parallel circuitis about half of the time of each half cycle of the applied voltage. Forexample, for a voltage source of 60 c.p.s.,.with a time duration of 8.3millisecends for each half cycle, a time constant of 4.5 millisecondsgiving a decay to about 30% in the condenser charge during each halfcycle is found to produce very satisfactory results. 'To this end theresistor 22 may have a value of the order of 60,000 ohms. Since theseries RC time constant of the condenser '17 and the resistor 18 is onlyof the order of .27 millisecond as a typical value, a representativevalue for the parallel RC time constant is of the order of sixteen timesthat of the series RC time constant. v

An observance of the current pulses through the ultraviolet tube 10shows that they decay exponentially in the manner of discharge currentsthrough 'RC circuits. The point in each half Voltage cycle at which theultraviolet tube fires depends upon the intensity of the radiation excitation. If the excitation is high the current pulses start during theearly part of each voltage wave and the peak amplitude of the currentislow. If the excitation is low the current pulses tend to start when thevoltage is at its peak value and the peak value of the current istherefore relatively high,

A function of the condenser 21 in parallel with the relay 20 is not onlyto smooth out the current pulses into a steady current but also to actas an integrating means so that aseri'e's of successive pulses isrequired to operate the relay. The condenser 21 therefore serves toprevent occasional single pulses from operating the relay.

In FIGURE 2 there is shown an operating circuit for the ultraviolet tube10 wherein the operation is from unidirectional voltage pulses insteadof from an AC. voltage source. This embodiment of the invention is nowcovered by our divisional application Serial No. 274,272 filed April 19,1963. Connected in this case across the secondary winding of thetransformer 16 is a circuit 23 serially including a half-Way rectifier24, resistor 25, the ultraviolet tube 1d and the relay 20 and condenser21 in parallel. The resistor 25 is chosen to provide a current flowwhich will extend the glow discharge throughout the Working region ofthe ultraviolet tube When the tube is triggered by incident ultravioletradiation. There being no condenser in the circuit 23 the triggereddischarge current flows at a rate dependent upon the applied potentialon the circuit 23 and the resistance 25, the current flow continuinguntil the voltage supplied to the ultraviolet tube has fallen belowabout 330 volts. A typical value for the resistor 25 is of the order of6,000 ohms. Preferably, the resistor 25 is connected in the circuit inclose physical relationship to the ultraviolet tube 19 so as to preventa flow of stray capacity currents which may quench the ultraviolet tubeprematurely and prevent operation of the relay 20. Although the currentfiow during each discharge pulse does not drop greatly from its initialvalue, and may even pass through a peak value, before the tube isquenched the greater duration of the current at or near its peak valvedoes not produce overheating of the electrodes because the current flowsonly during alternate half cycles.

The embodiments of our invention herein particularly described areintended to be illustrative and not limitative of our invention sincethe same is subject to other changes and modifications without departurefrom the scope of our invention, which we endeavor to set forth in thefollowing claims.

We claim:

ll. An ultraviolet detector system comprising an ultraviolet detectortube having a sealed glass envelope, an ionizing gas and a pair ofelectrodes having intermediate portions in an adjacent relationship anddiverging portions supported at their ends to provide a working regionnot .embracing the supported end portions and wherein the emission of anelectron from incident ultraviolet radiation is capable of triggeringthe tube into a glow discharge when a striking potential is appliedacross the electrodes, said tube being characterized in that an appliedvoltage of a predetermined value will extend the glow dischargethroughout said working region to clean the electrodes of contaminantsfrom said glass envelope tending to change the spectral response of thetube, and an operating circuit for said detector tube including autilization device responsive to flow of discharge current in said tubewhen the tube is triggered by ultraviolet radiation and a source ofvoltage providing successive voltage pulses across said tube of whicheach pulse reaches at least said predetermined value to cause the glowdischarge to extend throughout the working region.

2. The ultraviolet detector system set forth in claim 1 wherein saidvoltage source is A.C. and wherein said opertaing circuit includes acondenser connected eifectively in series with said voltage source,detector tube and utilization device.

3. The ultraviolet detector system set forth in claim 2 wherein saidoperating circuit includes a resistor and condenser connected in serieswith said detector tube and having an RC time constant adapted to limitthe R.M.S. value of the successive current pulses to a levelinsufficient to cause overheating of said electrodes when the detectortube is subjected to utraviolet radiation of a saturation level.

4. The ultraviolet detector system set forth in claim 3 wherein said RCtime constant is of the order of of the time period of each pulse ofapplied voltage of said source.

5. The ultraviolet detector system set forth in claim 3 including aresistor connected in parallel with said condenser for bleeding off aportion of the charge on said condenser between successive currentdischarge pulses in said detector tube.

6. The ultraviolet detector system set forth in claim 5 wherein the RCtime constant of said condenser and parallel resistor is of the order ofone-half the time period of each half-cycle of said A.C. source ofvoltage.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON,Primary Examiner. JAMES W. LAWRENCE, Examiner.

1. AN ULTRAVIOLET DETECTOR SYSTEM COMPRISING AN ULTRAVIOLET DETECTORTUBE HAVING A SEALED GLASS ENVELOPE, AN IONIZING GAS AND A PAIR OFELECTRODES HAVING INTERMEDIATE PORTIONS IN AN ADJACENT RELATIONSHIP ANDDIVERGING PORTIONS SUPPORTED AT THEIR ENDS TO PROVIDE A WORKING REGIONNOT EMBRACING THE SUPPORTED END PORTIONS AND WHEREIN THE EMISSION OF ANELECTRON FROM INCIDENT ULTRAVIOLET RADIATION IS CAPABLE OF TRIGGERINGTHE TUBE INTO A GLOW DISCHARGE WHEN A STRIKING POTENTIAL IS APPLIEDACROSS THE ELECTRODES, SAID TUBES BEING CHARACTERIZED IN THAT AN APPLIEDVOLTAGE OF A PREDETERMINED VALUE WILL EXTEND THE GLOW DISCHARGETHROUGHOUT SAID WORKING REGION TO CLEAN THE ELECTRODES OF CONTAMINANTSFROM SAID GLASS ENVELOPE TENDING TO CHANGE THE SPECTRAL RESPONSE OF THETUBE, AND AN OPERATING CIRCUIT FOR SAID DETECTOR TUBE INCLUDING AUTILIZATION DEVICE RESPONSIVE TO FLOW OF DISCHARGE CURRENT IN SAID TUBEWHEN THE TUBE IS TRIGGERED BY ULTRAVIOLET RADIATION AND A SOURCE OFVOLTAGE PROVIDING SUCCESSIVE VOLTAGE PULSES ACROSS SAID TUBE OF WHICHEACH PULSE REACHES AT LEAST SAID PREDETERMINED VALUE TO CAUSE THE GLOWDISCHARGE TO EXTEND THROUGHOUT THE WORKING REGION.